To clarify my position they do technically stack but they stack in a way that gives no benefit for casting more then one plant growth using the enrichment option. The first casting raises the potential productivity over the course of the next year to one-third above normal, but the spell does not say that it changes what the normal value is and the new value can reasonably be said to not be normal since presumably plants do not normally have plant growth cast on them. So the second casting also raises the potential productivity over the course of the next year to one-third above normal, but this is the same potential productivity value as the first casting of the spell gave since the second casting calculates from the same value of normal. Thus the second casting while technically stacking did not change the productivity of the plants.